This invention relates to modulation systems and methods and more particularly to systems and methods that can efficiently modulate a signal onto a radio frequency carrier.
Modulation systems and methods are widely used in transmitters to modulate information including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators are also referred to as xe2x80x9cmixersxe2x80x9d or xe2x80x9cmultipliersxe2x80x9d. For example, in a mobile radiotelephone, a modulator is used in the radiotelephone transmitter.
In modern radiotelephone communications, mobile radiotelephones continue to decrease in size, cost and power consumption. In order to satisfy these objectives, it is generally desirable to provide modulation systems and methods that can provide high power modulation while reducing the amount of battery power that is consumed. Unfortunately, the power amplifier of a modulator may consume excessive power due to efficiency limitations therein. More specifically, it is known to provide linear Class-A or Class-AB power amplifiers that may have efficiencies as low as 30% or less. Thus, large amounts of battery power may be wasted as heat.
In modulation systems and methods, it is known to provide a desired modulation of a radio signal as a stream of complex numbers including a real part and an imaginary part. Such a stream of complex numbers may be generated by a digital signal processor in a radiotelephone. Systems and methods for modulating a stream of complex numbers are described in U.S. Pat. No. 5,815,531 to the present co-inventor Dent entitled xe2x80x9cTransmitter for Encoded Data Bitsxe2x80x9d, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference.
As described in the ""531 patent, Quadrature Amplitude Modulated signals are generated from data bits by using a first Quadrature Phase Shift Keying (QPSK) modulator for encoding a first pair of the data bits into one of four carrier signal phases, thereby producing a first QPSK signal. A second QPSK modulator encodes a second pair of the data bits into one of four carrier signal phases, thereby producing a second QPSK signal. The first QPSK signal is amplified to a first power level, and the second QPSK signal is amplified to a second power level. The first and second amplified signals are then combined to produce a signal in which four data bits are encoded. Offset Quadrature Phase Shift Keying (OQPSK) may be used in place of the first and second QPSK modulators, so that an Offset Quadrature Amplitude Modulation (OQAM) transmitter is formed. An OQPSK modulator encodes data bits by encoding a first sub-group of the data bits into a real part of a complex signal at an odd instant of a clock, and by encoding a second sub-group of the data bits into an imaginary part of the complex signal at an even instant of the clock. OQPSK modulation provides the benefit of having all signal transitions being constrained to trajectories around constant radius circles, thereby producing spectral efficiency. See the ""531 patent abstract.
Notwithstanding the improvements of the above described patent, there continues to be a need for improved methods and systems for converting a stream of complex numbers representing a desired modulation of a radio signal into a modulated radio power signal at a radio carrier frequency. Preferably, these systems and methods can perform conversion at high efficiencies so that the size, cost and/or power consumption of the modulation system may be reduced.
It is therefore an object of the present invention to provide improved modulation systems and methods.
It is another object of the present invention to provide modulation systems and methods that can provide high efficiency.
It is yet another object of the present invention to provide modulation systems and methods that can modulate a stream of complex numbers representing a desired modulation of a radio signal, at high efficiency.
These and other objects are provided, according to the present invention, by converting a stream of complex numbers into polar form, including an amplitude-representative part and a phase-representative part. The amplitude-representative part of each of the converted complex numbers is represented as a plurality of digits of decreasing numerical significance. The phase-representative part of each of the converted complex numbers is phase-modulated at the radio carrier frequency to produce a phase-modulated drive signal. The phase-modulated drive signal is amplified in a plurality of power amplifiers. A respective one of the power amplifiers provides a contribution to a modulated radio power signal at a respective amplifier output, that is related to a respective one of the digits of decreasing numerical significance. The output amplitude of a respective one of the plurality of power amplifiers is controlled by applying a respective one of the plurality of digits of decreasing numerical significance to a respective one of the plurality of power amplifiers. The amplifier outputs of the plurality of power amplifiers are combined to form the modulated radio power signal.
The output levels of the plurality of power amplifiers may be combined using various techniques. For example, a respective output power level may be applied to a primary winding of a respective one of a plurality of transformers, the secondary windings of which are series coupled to produce the modulated radio power signal. Alternatively, a respective output power level may be applied to a respective quarter wave transmission line, and the quarter wave transmission lines may be coupled to one another to produce the modulated radio power signal.
The digits may be binary digits such that the amplitude of a respective one of the plurality of amplifiers is controlled by turning a respective one of the plurality of power amplifiers on or off by applying a respective one of the plurality of digits of decreasing numerical significance to a respective one of the plurality of power amplifiers, the output impedance of an xe2x80x9coffxe2x80x9d amplifier being arranged to be zero or very low. The amplitude-representative parts of the complex numbers may have both positive and negative values. In this case, the output amplitude of a respective one of the plurality of power amplifiers is controlled to be either a maximum positive amplitude or a maximum negative amplitude by applying a respective one of the plurality of digits of decreasing numerical significance to a respective one of the plurality of power amplifiers.
Alternatively, the digits may be ternary digits. Then, the output amplitude of a respective one of the plurality of power amplifiers may be controlled to be either a maximum positive amplitude, a maximum negative amplitude or zero by applying a respective one of the plurality of ternary digits of decreasing numerical significance to a respective one of the plurality of power amplifiers. When zero is selected, the output impedance of the amplifier is arranged to be zero or very low.
According to another aspect of the present invention, the stream of complex numbers is converted into polar form including an amplitude-representative part and a phase-representative part. The amplitude-representative part is represented as a plurality of digits of decreasing numerical significance. The phase-representative part of each of the converted complex numbers is modulated at the radio carrier frequency, to produce a phase-modulated drive signal. The phase-modulated drive signal is power-amplified in a plurality of power amplifiers. A respective power amplifier provides an output voltage level at a respective amplifier output. A respective one of the plurality of digits of decreasing numerical significance is applied to a respective one of the plurality of power amplifiers to thereby control the output amplitude of a respective one of the plurality of power amplifiers. The amplifier outputs of the plurality of power amplifiers are combined according to a combining ratio to produce the modulated radio power signal. Either the output voltage levels or the combining ratio or both are selected such that the modulated radio power signal is related to the numerical significance of the plurality of digits.
In a first embodiment, each of the power amplifiers produces the same output voltage level and the combining ratio is selected such that the outputs of the power amplifiers are combined according to a ratio that is related to the numerical significance of the digits that are associated with the respective power amplifier. In another alternative, the combining ratio is unity and the output voltage level of a respective power amplifier is related to the numerical significance of the digits that are associated therewith. In yet another alternative, both the amplifier output level and combining ratio are selected such that the modulated radio power signal is related to the numerical significance of the plurality of digits. The amplifier outputs may be combined as was described above. The digits may be binary digits or ternary digits as was described above.
Accordingly, the present invention can convert the stream of complex numbers representing a desired modulation of a radio signal into an amplitude and phase-modulated radio power signal at a radio carrier frequency at high efficiency. Reduced size, cost and/or power consumption of a mobile radiotelephone or other transmitter may be obtained. Power modulation systems and methods may be provided.